It is often necessary or desirable to separate gases such as oxygen and nitrogen from each other or from other gas mixtures. The utility of this can range from a need to remove oxygen from a stream of combustible gases, to remove nitrogen to increase the oxygen content of medical oxygen, to perform a separation in a petrochemical process or as part of a process for the production of pure gas streams. Another important gas separation is to remove carbon dioxide from natural gas.
Several techniques, including absorption into a solvent phase, pressure swing adsorption (PSA) and membrane separation have been developed for the purpose of performing these separations. Membrane separations are based upon the relative permeability of one gas molecule being much higher than that of another gas molecule through an otherwise impermeable membrane. Polymeric membranes have attracted a great deal of interest for use in gas separation. Some membranes which are used include silicone membranes for oxygen/nitrogen separation, cellulose acetate membranes for carbon dioxide removal from natural gas and silicone-coated polysulfone membranes for hydrogen recovery from various waste streams. In a typical operation, a pressure differential is maintained across the polymeric membrane to provide the driving force for the permeation. Two properties of the membrane are of critical importance in determining the performance characteristic, which is possessed by the membrane. The first property is the solubility of the gas in the membrane, while the second property is the diffusivity of the gas in the membrane material. The product of these two properties, that is, solubility and diffusivity, is called the permeability. The higher the membrane permeability, the more attractive is the use of membranes for a gas separation process.
The separation of a gas mixture utilizing a membrane is effected by passing a feed stream of the gas across the surface of the membrane. Since the feed stream is at an elevated pressure relative to the effluent stream, a more permeable component of the mixture will pass through the membrane at a more rapid rate than will a less permeable component. Therefore, the permeate stream which passes through the membrane is enriched in the more permeable component while the residue stream is enriched in the less permeable component of the feed.
There are various references which disclose the incorporation of various materials within separation membranes. U.S. Pat. No. 3,457,170 to Havens; U.S. Pat. No. 3,878,104 to Guerrero; U.S. Pat. No. 3,993,566 to Goldberg et al; U.S. Pat. No. 4,032,454 to Hoover et al; and U.S. Pat. No. 4,341,605 to Solenberger et al teach the use of structural supports or reinforcement fibers or fabrics to aid the membrane in resisting the high pressures used in the reverse osmosis process. U.S. Pat. No. 3,556,305 to Shorr shows a “sandwich” type reverse osmosis membrane comprising a porous substrate covered by a barrier layer, in turn covered by a polymer or film bonded to the barrier layer by an adhesive polymeric layer. U.S. Pat. No. 3,862,030 to Goldberg shows a polymeric matrix having an inorganic filler such as silica dispersed throughout which imparts a network of micro-voids or pores of about 0.01 to about 100 microns, capable of filtering microscopic or ultrafine particles of submicron size. U.S. Pat. No. 4,302,334 to Jakabhazy et al discloses a membrane “alloy” comprising a hydrophobic fluorocarbon polymer blended with polyvinyl alcohol polymer which imparts hydrophilic properties to the membrane.
U.S. Pat. No. 4,606,740 describes the composition and manufacture of multi-component membranes containing polyethylene glycol useful in the separation of gas mixtures. This reference also discloses the use of polysulfone supports. U.S. Pat. No. 5,127,925 describes the separation of gases by use of a mixed matrix membrane consisting of polysilicone having a solid particulate adsorbent incorporated therein. U.S. Pat. No. 6,726,744 teaches the separation of gases by using a mixed matrix membrane comprising polyethylene glycol, silicone rubber and activated carbon on a porous support. Improvements in selectivity and permeability, particularly in connection with the separation of methane and carbon dioxide are still highly desirable.
We have now discovered a novel and highly mixed matrix membrane that is particularly useful in the separation of carbon dioxide from methane and carbon dioxide from nitrogen. We have further developed a specific combination of ingredients making up a mixed matrix membrane which heretofore has not been known prior to our invention.